Objective beach-state classification from optical sensing of cross-shore dissipation profiles

Remote sensing using terrestrial optical CCD cameras is a useful data-collection method for geophysical measurement in the near-shore zone, where in situ measurement is difficult and time consuming. In particular, optical video sensing of the variability in the visible spectrum from the sea surface due to the near-shore incident wave field is becoming an established method for distal measurement of near-shore sub-tidal morphology. We report on the use of a low-mounted shore-normal camera for gathering data on cross-shore dissipative characteristics of a dynamic, open beach. Data is analysed for the purposes of classifying three of Wright and Shorts’ intermediate classes of morphological beach state, as determined by expert raters. Although these beach states are usually thought of as being distinctive in terms of their longshore bar variability, theory predicts that differences should also be observed in cross-shore dissipative characteristics. Three methods of generating features from statistical features from the archived optical data are described and compared, in terms of their ability to discriminate between the beach states. The best performance was obtained using an pixel intensity percentile representation (which does not assume a Gaussian intensity distribution), which classified 85% of the 284 cases correctly. Class centre moment profiles for each beach state were constructed, and results indicate that cross-shore wave dissipation becomes more disorganized as linear bars devolve into more complex transverse structures.